Last week I finally completed my Zkit 4 & Zkit 5.1 setup with a 12AU7 tube.and have been breaking it in ever since, playing it 4-5 hours non-stop everyday.The sound keeps getting better as each day goes by...I have been looking into trying it out with a 6dj8/6922 tube, and according to the info posted on the Zkit 4 product page, all I needed to do was to get the heater voltage down to 6.3V...

My question is 2 folds:

- With the 15 Ohm resistor placed in series with the heater load, the tube filament is actually operating in a starved voltage setup (around 10.5 Volts instead of the full 12.6 Volts).So my first question is what's a safe starved operating voltage for the 6DJ8 family of tubes?

- Looking at the 12AU7 and 6DJ8 tube pinout, and internal topology, it turns out pin 9 on the 12AU7 tubes is used as a center tap for both heaters.In which case I assumed that i could run my heater voltage at a constant 6.3 volts (or whatever the safe starved voltage is) and wire the connections through a switch, in a way such that will give these two options:

When i'm using a 6DJ8 tube, the connection is made as per the schematics (going directly to pins 4 and 5 of the tube.

When i'm using the 12AU7, first power supply wire will be connected to pin 9 of the tube, while pins 4 and 5 will be connected together and merged into the second power supply wire.(hope i'm clear enough, if not, let me know and i'll draw a sketch of what I have in mind)Could this work, or are there any drawbacks? I have no previous experience with tubes, but basic electrics say this should work...